It is well known that laser beams may in some circumstances damage optical materials and coatings. Referred to as laser induced damage (LID), it is believed that damage to optical materials may stem from the direct interaction of photons with the material. In particular, it is believed that the electric field component of the laser radiation may interact with surface defects, leading to thermal breakdown. The decomposition of the material may lead to pitting or carbon formation, which in turn may lead to increased material damage and rapid failure of the optic.
To mitigate LID, optical materials that are relatively tolerant to laser damage may be selected for use in laser technologies. After the optical materials are selected, screening tests that are specific to a particular system are conducted. Typically, the tests involve exposing an optic to laser radiation until damage occurs, and then repeating the tests multiple times to collect statistically meaningful information about the laser induced damage threshold (LIDT) of the optic. The LIDT of the optic is defined to be the number of pulses required to damage the optic, and is measured for a particular coating and optical material against a laser source that is representative of the hardware design (e.g., having the same fluence, wavelength, and pulse width). In some conditions, laser optic materials routinely survive a billion pulses or more.
One known cause of LID to an optic is molecular and particulate contamination. This contamination may result, for example, from outgassing products that condense onto the optic. Outgassing is the slow release of a gas that was frozen, trapped, absorbed, or adsorbed in some material. Common sources of gas include moisture, sealants, lubricants, plastics, and adhesives, but even metals and glasses can release gases from cracks or impurities. Contaminants, including outgassing products, may degrade optics by causing light transmission loss, increased light scatter, and/or obscuration. While the body of research on these contamination effects extends over multiple decades, a relatively new phenomenon of contamination enhanced laser induced damage (CELID or C-LID) has only recently gained attention.
CELID is generally observed when a laser and associated optics are enclosed in either a vacuum or sealed gas environment (typically nitrogen or air). As such, CELID is of particular concern during the development of space-based optical systems. Such optical systems are often developed first as a benchtop version that is a functional representation of the space-based version. The benchtop version, however, is not fully analogous to the space-based version, because the benchtop version is typically not enclosed in either a vacuum or sealed gas environment. Therefore, it is common for the benchtop version not to exhibit laser induced damage, and it is typically not until the space-based version is built and enclosed, presumably with the same optical design, that CELID becomes an issue. For example, it is believed that an open-environment benchtop version may not exhibit CELID because the contaminants, particularly molecular species, cannot build up in significant concentrations due to open circulation throughout the optical cavity of the benchtop system. In contrast, it is believed that the space-based version may exhibit CELID because of its vacuum or sealed gas environment. The implication is that standard practices for designing, building, and operating benchtop system to prevent CELID may not be fully applicable to space-based optical systems when operated in their flight enclosure, e.g., in a vacuum or sealed gas environment.
CELID may cause laser power to rapidly decay and lead to premature failure of optics. Encountered during the development of space-based lasers, such as the ones included on the National Aeronautics and Space Administration (NASA) Mars Orbiter Laser Altimeter (MOLA) and Geoscience Laser Altimeter System (GLAS) missions, CELID has also been observed in laboratory studies. In these reports, optics expected to survive well over 1 million pulses from an infrared laser were observed to fail in as few as 8,000 pulses when contamination was observed to be present.
Certain types of contaminants have been observed to cause CELID, resulting in accelerated damage to optics. The most common contaminants include hydrocarbons and silicones. The most widely studied contaminant for CELID is toluene, also known as methylbenzene. Toluene, a common outgassing compound of epoxies, is relatively volatile, and is a common chemical that has a similar chemical structure to a number of other aromatic hydrocarbon contaminants. Toluene has been observed to induce damage on optics, while some other contaminants such as acetone, a common optics cleaning solvent, have not been observed to induce similar damage.
Several previously-known systems attempt to address CELID. U.S. Pat. No. 5,770,473 to Hall et al. discloses a package for a high power semiconductor laser that includes a hermetically sealed container filled with a dry gaseous medium containing oxygen, for example air having less than 5000 ppm water. Hall discloses that the oxygen within the packaging atmosphere serves the important function of minimizing laser damage by organic impurities. Hall discloses that there is a downside to using oxygen, namely, that it can react with hydrogen to form water within the laser enclosure. Additionally, Hall discloses that the water, in turn, can adversely affect the overall operation of the electronic components within the enclosure, including the semiconductor laser, by, for example, creating a short circuit between the conductors which interconnect the components. Hall discloses that the use of a getter material that adsorbs or absorbs water in addition to organic impurities, such as porous silica and various zeolites, can help to minimize this problem.
Schröder et al., Investigation of UV Laser Induced Depositions on Optics Under Space Conditions in Presence of Outgassing Materials, 6th Int'l Conf. on Space Optics, held 27-30 Jun. 2006 at ESTEC, Noordwijk (2006) discloses that the outgassing of organic material under vacuum conditions combined with high laser fluences can lead to formation of deposits on the optics. Specifically, Schröder discloses an investigation of UV-laser induced deposits on uncoated fused silica optics in a test chamber under simulated space conditions in the presence of outgassing materials. Schröder discloses the use of a Nd:YAG laser and epoxy, silicone, and polyurethane contaminants in the investigation. Additionally, Schröder discloses that for testing the influence of water on the formation of deposits a liquid reservoir with about 50 ml was connected via a needle valve to the chamber, and that the partial pressure of the water vapor in the chamber was measured with a gas type independent capacitance sensor. Schröder discloses a test with an epoxy-based contaminant at a partial pressure of 5 mbar water and compared it with a test without water and stated that water reduced deposit formation significantly.